This invention relates to apparatus for closing off the flow of hydraulic fluid when flow rate exceeds a pre-set value, and more particularly to such a velocity fuse in which the pre-set value can be adjusted while the fuse is in the hydraulic line, and the apparatus is not triggered by high flow rates of very short duration.
In many hydraulic systems a line rupture causes a greatly increased flow rate, as fluid is lost. The loss of fluid may adversely affect function that can be saved if flow is promptly cut off. For this purpose, it is well known to employ excess flow protectors called velocity fuses. U.S. Pat. No. 4,383,549 issued May 17, 1983 to Maldavs discloses a poppet valve device of this type in which the flow rate required to actuate shut off is adjustable while the device is connected in the circuit. This is convenient in that the set point may need to be adjusted as the fluid heats up and viscosity is reduced. As it points out, such devices in the past have had the problem of being actuated by brief surges of flow, such as at start up. Its solution to this problem is to make the device with a dash pot chamber to prevent premature closing of the poppet valve from high flow surges of short duration. It has two fluid-filled chambers 31 and 32 that communicate through an annular constriction. As the poppet moves to close the valve, fluid must move from chamber 31 through the constriction to chamber 32. This takes time. If the surge has stopped before the valve closes, the spring returns the poppet to full open. The adjustment of the flow rate set point is achieved by moving the poppet closer to its closing off seat by an external set screw. Although this is convenient, the structure is very complex to manufacture. An additional problem is encountered with this structure. The damping function is affected by this flow rate adjustment. As the poppet is moved closer to its seat for lowering the set point, fluid is moved into chamber 32 from chamber 31. Now there is less time required to fill chamber 32. Consequently, the device is now protective of only shorter duration surges.
It is accordingly an object of the invention to provide a hydraulic fuse that is simple of construction, less expensive to manufacture, and having a flow rate set point that is adjustable while the device is connected in the hydraulic circuit. It is another object that the device be provided with a damping mechanism that is unresponsive to flow rate surges that are of short duration. It is yet another object that the set point adjustment not affect the damping mechanism over most of the set point adjustment range. These and other objects, features, and advantages of the invention will become more apparent when the detailed description is studied in conjunction with the drawings in which like elements are designated by like reference characters in the various drawing figures.